1. Field of the Invention
The invention relates to a method for testing a spring-loaded safety valve that includes a closing device and is acted upon by a medium. The invention also relates to an apparatus for testing such a spring-loaded safety valve.
The closing device of the spring-loaded safety valve has a spindle onto which a valve plate is formed. The valve plate may rest on a valve seat, which is a component of a valve housing. In that situation, the safety valve is closed. In other words, a connection between an incoming and an outgoing line for the medium is closed.
Such spring-loaded safety valves are used, for instance, to control a main valve but are also used in a self-shielding fashion. For instance, such a safety valve may communicate on the inlet side with the primary loop of a nuclear power plant. If the pressure on the inlet side exceeds a response pressure of the safety valve, the closing device of the safety valve is open counter to a spring force. The medium can then pass through the safety valve to reach a main valve and open it. It is not until the pressure of the medium in the safety valve has dropped below a closing pressure of the safety valve that the spring force is overcome again and the safety valve is closed.
The response pressure of a spring-loaded safety valve is always above the closing pressure. The valve opens if the higher response pressure is exceeded and does not close again until the pressure again drops below the lower closing pressure.
In the course of recurring safety checks, particularly in a nuclear power plant, the mode of operation of the spring-loaded safety valves, for instance of the control valves, must also checked. It is known to perform such a test at a pressure of the medium that is above the closing pressure of the safety valve. It is then possible to determine the instantaneous closing pressure of the safety valve, as is required in a comprehensive safety check. In that kind of known test, the pressure of the medium is reduced after the opening of the safety valve, by cooling the medium, until such time as the safety valve closes. For example, the cooling may be performed by spraying with cold water. However, that leads to a high thermal strain on the components of the valve and other components located in the loop of the medium.
Due to that high thermal strain, testing of the safety valves was previously frequently performed at a pressure of the medium which was below the closing pressure. In that case, an auxiliary lifting device was needed to enable opening of the safety valve. Such an auxiliary lifting device is also necessary if a test is done with a pressure of the medium that is between the closing pressure and the response pressure.
A typical auxiliary lifting device is an appliance that can be mounted on the safety valve. On one hand the housing of the auxiliary lifting device is joined to the valve housing, and on the other hand a lifting bell in the auxiliary lifting device is joined to the spindle of the safety valve. Such a lifting bell may also be constructed to be flat, in the form of a lifting plate. In order to open the safety valve, the auxiliary lifting device is acted upon, through a separate supply system, with an auxiliary medium which may be nitrogen. The auxiliary medium lifts the lifting bell and thus reinforces the opening of the spring-loaded safety valve. Once the auxiliary lifting device is relieved of the auxiliary medium, the spring force of the safety valve acting in the closing direction immediately predominates, so that the valve closes rapidly. In that rapid motion, however, it is not possible to detect the closing pressure.
Testing of a safety valve below the closing pressure is also believed by professionals in the field to offer no likelihood of success, because in a safety valve that is used, for instance, as a control valve, complex flow conditions prevail due to its construction. Heretofore, ascertaining the closing pressure of a control valve has therefore not been done as a rule.